Technical Field
Embodiments of the subject matter disclosed herein generally relate to investigating an underground formation by simultaneously measuring its seismic and electromagnetic response to excitations or, more specifically, to jointly recording seismic and electromagnetic data.
Discussion of the Background
Reflection seismology is a popular method of exploring an underground formation by injecting seismic excitations into the formation and measuring the formation's response to them. Electromagnetic methods have been used to provide complementary information to information extracted using seismic methods. Electromagnetic methods are particularly efficient in differentiating rock layers that contain hydrocarbons from other, non-oil-bearing rock layers due to the large resistivity contrast. Contrasts of 1:2 or even 1:10 are common between an oil-saturated and a brine-saturated reservoir rock.
Conventionally, seismic and electromagnetic surveys are performed at different times, with electromagnetic data acquisition system being deployed and operated independently from seismic data acquisition.
Recently, time-lapse surveys have been used with increasing frequency to track the evolution of underground formations including hydrocarbon reservoirs by exploring their structure at long time intervals of months or even years. In order to minimize the changes of data acquisition systems that may be difficult to differentiate from the sought-after underground formations' changes, receivers and sources are sometimes kept in place for months or even years above the underground formation between surveys. In this context, it is necessary to deploy electromagnetic data acquisition while the seismic data acquisition system remains in place or vice-versa. Deployment and/or retrieval of equipment without damaging a co-located seismic or electromagnetic survey setup may be challenging and therefore time consuming.
Conventional seismic systems using moving coil sensors and wire-based data transmission (analog or digital) are affected by interference due to electromagnetic sources used for electromagnetic exploration of the subsurface. Therefore, traditionally, the seismic and electromagnetic surveys have been performed at different times. Consecutive seismic and electromagnetic surveys require substantial time during data acquisition and then during processing to properly combine complementary seismic and electromagnetic information.
Accordingly, it is desirable to provide a data acquisition system for both seismic and electromagnetic surveys that would shorten overall deployment, data acquisition and data processing times, while facilitating combination of complementary seismic and electromagnetic information about the investigated underground structure and avoiding the electromagnetic interference problems of conventional systems.